The reaction stage in the nitration of toluene provides an isomer mixture comprising ortho-, meta- and para-nitrotoluene and also dinitrotoluene isomers and further components. The technical task is to remove the o-, m- and p-nitrotoluene isomers from this mixture and to obtain them in a high purity of at least 99.70%. This is done in a series of distillation columns connected in series, some of which are also connected together via circulation streams. As a consequence of the small boiling point differences of the nitrotoluene isomers to be obtained, distillation columns having a high number of plates and high reflux ratio are required for separation. In particular, the separation of m- and p-nitrotoluene whose boiling point difference is only about 3 K requires a high number of plates. For this reason, these isomers are separated in a column which, in terms of apparatus, is divided into two adjacent column sections having a total of one evaporator and one condenser. Owing to the low temperature sensitivity, it is impossible to use a column temperature as an auxiliary controlling parameter to control the concentration of p-nitrotoluene which is obtained at the bottom of the second column section.
Since column temperatures cannot be used as auxiliary control parameters to maintain the product specification of p-nitrotoluene, the process can be controlled by the use of online near infrared spectroscopy instruments. However, it is found that their use for analysis of the end product, i.e. at the liquid phase exit or column, has two distinct disadvantages. These are:
When the concentration of the end product (p-nitrotoluene) is measured, the NIR analysis provides measurements which, owing to the high concentration of p-nitro-toluene, do not have the required precision.
Even if it were possible to achieve the precision required for the process control with the analysis, the mounting of the measuring instrument in the end product stream is not viable, since concentration deviations are registered there too late and therefore counteracted too late.
Owing to the product properties, considerable contamination and discoloration in the end product stream is to be expected during the startup procedure of the process. This rules out the use of spectroscopy as an analytical method for the control of the process.
In-house investigations have shown that the highest sensitivity in the product stream in the event of concentration changes of the liquid phase product is between the two column sections. In the product stream mentioned, the concentration change is a factor of 10 higher than the resulting concentration change in the liquid phase product. Furthermore, a lower p-nitrotoluene concentration compared to the end product is present in the product stream between the columns. In this product stream, a higher precision is therefore achieved using the analytical instrument. The precision is approximately twice as high as in the case of a measurement in the end product.
The idea on which the invention is based is to accommodate the analytical instrument in the product stream between the two column sections. When the analytical instrument is then incorporated in a control circuit, this allows the concentration to be kept constant in the product stream between the two column sections. However, this does not ensure that the concentration of the end product removed at the bottom of the second column section also remains constant. The reason for this is that the separating performance of the second column section which is between the point at which the instrument is mounted and the column bottom is subject to constant changes as a consequence of control interventions and disruptive influences.
The object of the invention is achieved in accordance with the invention by coupling the online analytical instrument in the above-described arrangement with an online process model.